The present invention relates to multiplexed telemetering systems and, in particular, to low-power bio-telemetry systems for obtaining EEG and other physiological data.
In recent years, a number of telemetry systems have been developed for obtaining physiological data from conscious, unrestrained individuals or animals. For the most part, the development effort has been directed toward the problem of providing compact, lightweight and low-power equipment which can be mounted, for example, on the individual's head with the individual free to move about unencumbered by trailing wires and other disturbing influences. Examples of such prior systems are described in the following publications: "A Four Channel Integrated Circuit Telemeter for Seizure Monitoring", R. W. Vreeland and C. L. Yeager, Digest of 7th International Conference on Medical and Biological Engineering, 1967, Stockholm, Sweden; "A Compact Six-Channel Integrated Circuit EEG Telemeter", Vreeland, Yeager and Henderson, Jr., Electroencephalography and Clinical Neurophysiology, Elsevier Publishing Company, Amsterdam, 1971, 30:240-245; and "A Multichannel Implantable Telemetry System", Medical Research Engineering, March-April, 1969 by Fryer, Sander and Datnow.
Although these prior systems have been most helpful, there is a continuing need for improvement. In particular, it is highly desirable to provide bio-telemetric systems which can be physically mounted on the subject to be studied and which then are capable of continuously operating for unusually long periods of time without any need for battery changes or other similar maintenance. As will be appreciated, the desire for a lengthy period of undisturbed operation is based upon the benefits which result when the subject of the study is permitted to function throughout the entire period in an undisturbed and unrestrained manner. Obviously, the long periods require the development of systems having unusually low-power consumption. In particular, the power consumption should be such that the need for battery changes can be avoided at least for overnight periods and, preferably, for periods extending for several or more days. Aside from the need for the longer operating periods, other recognized needs include the provision of more available channels for the studies as well as a reduction in the size of the instrumentation and the simplicity of its circuitry. In conventional multi-channel systems one of the factors contributing to power consumption is a separate amplifier for each of the channels with the multiplexing being performed subsequent to the amplification. Such arrangements apparently have been considered necessary to obtain a suitable signal level for the multiplexing. As will be described, a feature of the present system is the use of a single amplifier for all channels. In other words, the present system permits multiplexing at the amplifier input. However, this type of multiplexing imposes another problem in that the gating pulses needed to establish the channels then may be coupled into the channels. This undesired coupling will be recognized as unacceptable when it is considered that, for example, the gating pulses may be in the order of 6 volts while the data sample signals which are to be measured and analyzed by the system may be about 100.mu.v. Such a signal to noise ratio effectively denies the production of any worthwhile data.
The present invention resolves this transient coupling problem by employing special complimentary, metal oxide semiconductor (C/MOS) switches to perform the multiplexing prior to amplification by a single amplifier. As is known, the C/MOS switches utilize and "N" channel FET in parallel with a "P" channel FET and the enabling or gating of these C/MOS switches requires pulses of opposite polarity. Because of the opposite polarity, the portions of the gating pulses that otherwise would be coupled into the channel are effectively cancelled. The invention further employs a particular pulse position modulation for the amplified output as well as other particular features which significantly reduce the power requirements. For example, the system employs a clock-controlled shift register to provide a synchronizing interval or spacing as contrasted with the need for a separate synchronizing pulse.